Turbo machines, such as gas turbine engines, have one or more turbine modules, each of which includes a plurality of blades and vanes for exchanging energy with the working medium fluid. Some of the vanes may be fixed and others may be variable, that is, rotatable between positions in the gas turbine engine. A typical vane known in the prior art is shown in FIG. 7 and comprises, generally, a trunnion portion (a) and an airfoil portion (b). The airfoil portion comprises a leading edge (d) and a trailing edge (e). The trunnion portion (a) has an enlarged button portion (f) proximate to a transition zone (g) between the trunnion and airfoil. The variable vane in operation is mounted for rotation about axis (c) so as to locate the position of the leading edge of the airfoil as desired. Generally, the variable vane is rotated through an angle of about 40xc2x0.
Because the vanes of a gas turbine engine operate in a hostile environment, they are subjected to significant stresses, both steady stress and vibratory stress. The design of variable vanes of the prior art are such that the transition zone (g) from the trunnion portion (a stiff section of the variable vane) to the airfoil portion of the vane (a flexible section of the variable vane) is subjected to high stresses which may lead to failure of the vane at the transition area and subsequent catastrophic damage to the gas turbine engine.
Naturally, it would be highly desirable to provide a vane configuration which would reduce stress in the transition zone between the stiff portion (the trunnion) and the flexible portion (the airfoil) and provide a substantially smooth and continuous reduction in stress at the transition zone from the trunnion portion to the airfoil portion.
Accordingly, it is a principal object of the present invention to provide a vane which has reduced stress at the transition zone between the stiff section (trunnion) of the variable vane and the flexible section (airfoil) of the vane.
It is a further object of the present invention to provide in the transition zone of a variable vane a smooth and continuous reduction in stress from the stiff (trunnion) portion to the flexible (airfoil) portion of the variable vane.
It is a still further object of the present invention to provide a variable vane useful in gas turbine engines which may be casted.
According to the invention, the vane is provided with a stress reducing undercut on the stiff portion (trunnion portion) of the vane approximate to the transition zone between the stiff portion and the flexible portion (airfoil portion) of the vane. The undercut reduces stress in the area of the transition zone between the stiff and flexible portions of the vane. The actual vane design is determined by the function of the vane in the engine. Consequently, the stress reducing undercut geometry is such as to optimize the stress reduction in the transition zone for any particular vane design and function in a gas turbine engine. Accordingly, the width, radius of curvature, depth, location from the transition zone and sidewall angles of the stress reducing undercut is parametrically adjusted so as to minimize stress at the transition zone between the stiff section and the flexible section of the vane. According to the present invention, a plurality of stress reducing undercuts may be provided on the stiff section of the vane proximate to the transition zone defined by the junction of the stiff section and the flexible section. If the vane is provided with trunnion portions on either side of the airfoil, stress reducing undercuts may be provided on one or both trunnion portions of the vane in an area proximate to the respective transition zones between the trunnion portions and the airfoil. In addition, one or more enlarged portions (buttons) may be provided on one or more of the trunnions adjacent the transition zones for receiving the undercuts.
The design of the vane in accordance with the present invention offers a number of benefits. Firstly, the provision of stress reducing undercuts, which allow for smooth and continuous reduction in stress at the transition zones of the vane, greatly reduces the need for thickened airfoils which are typically used to reduce the stresses at the transition zones. Thus, there is a weight savings in the vane design. Secondly, the design allows for the vane to be cast rather than forged as is currently the case which results in substantial cost savings in manufacture.